Single-chamber vacuum furnace with hydrogen quenching

ABSTRACT

A method for operating a hardening furnace with hydrogen cooling, including filling the interior of the furnace with workpieces, sealing the interior of the furnace and evacuating it, filling the interior of the furnace with an inert gas, convective heating the workpieces with circulation of the inert gas, evacuating the interior of the furnace, further heating the workpieces by radiant heating and maintaining a theoretical temperature, switching off the heating, filling the interior of the furnace with hydrogen under high pressure, and circulating the hydrogen for cooling the workpieces, evacuating the interior of the furnace and filling it with an inert gas up to approximately atmospheric pressure, and opening the furnace and unloading the workpieces.

The instant application should be granted the priority date of Sep. 23, 2005 the filing date of the corresponding German patent application 10 2005 045 783.5.

BACKGROUND OF THE INVENTION

This invention relates to a method for operating a heat treatment plant with a single-chamber vacuum furnace, and to a single-chamber vacuum furnace.

In conventional single-chamber vacuum furnaces with gas quenching, different heat treatment methods have hitherto been carried out. These methods include annealing, soldering, sintering, degassing, hardening of tool steels, high speed steels, hot and cold working steels, as well as sub-zero refrigeration and tempering.

In the future, methods for hardening low alloy steels and vacuum gas carburization will also be added to these heat treatment methods. The need to expand the range of application results from the sharp rise in the cost of heat treatment operations. The most significant factor influencing production costs is the number of operating hours of the furnace plants per year. A heat treatment operation may use special furnaces for both of the last-mentioned methods if long-term contracts have been signed with customers. For the remaining business, for which there are only short-term contractual obligations, considerable flexibility is required. This means that both of the last-mentioned methods should, if possible, be carried out using the same furnaces as those used in the standard methods already mentioned.

Low alloy steels have hitherto mainly been hardened in controlled atmosphere furnaces with oil quenching (so-called sealed quenching). The vacuum gas carburization is carried out in special multi-chamber systems in which oil baths or high pressure quenching stations with nitrogen or helium are used for quenching.

Gas quenching is preferred because there is less distortion on the hardened material when quenching in gas, and because no subsequent cleaning is required. The multi-chamber furnaces normally used for this, in the prior art, are very expensive and have been developed specially for the mass production of automotive supplier parts or similar parts. They lack the flexibility to be adaptable to changing objectives and tasks imposed on them. Moreover, the process must be controlled and monitored much better in the case of single chamber furnaces because during the process the workpieces must not be moved and it must therefore be possible to arrange measurement sensors directly on or in the workpiece capable of recording its actual temperature.

Standard single chamber vacuum furnaces currently operate at 10 bars of nitrogen for quenching, and attain the following lambda values in the material core of structural steel bolts: Lambda =0.35 for bolts 20 mm ø×40 mm long Lambda =0.65 for bolts 40 mm ø×80 mm long Lambda =1.50 for bolts 80 mm ø×160 mm long Lambda =2.35 for bolts 120 mm ø×240 mm long

Here the lambda value is the cooling time from 800° C. to 500° C., measured in seconds, divided by 100. These values for the cooling rate are much slower than those that can be achieved when quenching in the oil bath.

The current prior art is described in the periodical article by R. Hoffmann, H. Steinmann and D. Uschkoreit: Possibilities and limitations of gas cooling—HTM 47-1992, p. 2 ff. For hardening low alloy steels and for vacuum gas carburization the quenching rate must be considerably increased to values which have so far not been regarded as attainable in a single-chamber furnace.

In order to expand the range of application of single-chamber vacuum furnaces in the manner described above, it is therefore the object of this invention to obtain quenching rates which correspond to quenching in the oil bath.

SUMMARY OF THE INVENTION

This object is realized by a method for operating a single-chamber vacuum hardening furnace, including the steps of filling the interior of the furnace with workpieces, sealing the interior of the furnace and evacuating the same, heating the workpieces and maintaining a theoretical temperature, filling the interior of the furnace with hydrogen under high pressure, switching off the heating and circulating the hydrogen for cooling the workpieces, discharging the hydrogen and evacuating the interior of the furnace, filling the interior of the furnace with an inert protective gas up to approximately atmospheric pressure, and opening the furnace and unloading the workpieces. This object is also realized by a single-chamber vacuum furnace for carrying out such method, and including a safety zone that surrounds the furnace and which prevents walking or travelling in the immediate vicinity of the furnace, with the exception of the front side thereof, which is adapted to be opened by mechanical or electrical means.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in the following with reference to an exemplary embodiment and the drawing, in which:

FIG. 1: shows the basic time curve of temperature and pressure, and the type of gas supplied in the gas carburization of workpieces;

FIG. 2: shows the spatial arrangement of the essential plant components in an operation in a diagrammatic representation; and

FIG. 3: shows a hardening furnace suitable for the method according to FIG. 1 in a cross-section from the side.

DESCRIPTION OF SPECIFIC EMBODIMENTS

In FIG. 1 the time curve of the temperature inside a hardening furnace when a method according to the invention is carried out is plotted in curve 1. Curve 2 shows the progress of the pressure inside the hardening furnace over the time during which the method is carried out. Here the time scale, which generally represents a period of five hours from the beginning to the end of the process, is arranged on the horizontal X-axis. The temperature scale covers a temperature range from 0° C. to 1,200° C. The pressure scale is arranged on the right-hand side of the graph and indicates the pressure in bars absolute, going from 0 bar to 10 bars, 0 bar being the vacuum.

Underneath the graph it is illustrated which gases are conducted at what times into the device, and when vacuum is applied. This representation is described later in detail.

The temperature curve is first described over the process time of 5 hours. Curve 1 commences at room temperature, which is section 1 a on temperature curve 1. The heating is then switched on and brings the furnace to a temperature of approx. 1,050° along section 1 b. The required temperature range in the different carburization applications for which the furnace will be suitable is 800° C. to 1,100° C.

At the target temperature of 1,050° C. predetermined in this exemplary embodiment the furnace temperature is kept constant in section 1 c. Section 1 c is approximately one hour long. In section 1 d the furnace is cooled quickly, approximately within 20 minutes, from 1,050° to room temperature. There the temperature is then kept constant until the end of the process, i.e. until the workpieces are unloaded. This section is denoted by 1 e.

The pressure curve, which is illustrated in curve 2, initially commences at 1 bar, i.e. at ambient pressure. This corresponds to the air which is present inside the furnace when the hardening furnace is loaded. In section 2 a of the graph the furnace space is evacuated for a period of approx. 20 minutes. The air in the furnace space is removed before the heating is switched on so that no oxidation can take place. Instead the furnace space is flooded with approx. 2 bars of nitrogen, as protective gas, when the heating is switched on, i.e. in the transition from 1 a to 1 b on the temperature curve. The pressure is maintained for a period of approx. 2 hours, which corresponds to section 2 b on the pressure curve. The nitrogen filling of the furnace is maintained up to a temperature of 700° C. Up to this temperature range the workpieces in the furnace are heated by convection heating. The furnace interior is then evacuated by the application of vacuum. The associated pressure reduction from 2 bars to 0 bar is identified by section 2 c. Further heating of the workpieces from 700° to the final temperature of 1,050° is achieved by radiant heating.

After the vacuum has been achieved in the furnace space, denoted by 2 d, a gas containing carbon is repeatedly introduced into the furnace space at a pressure of approx. 30 mbars for a short time. This gas, e.g. acetylene, loads the surface with carbon by thermal decomposition on the surface of the workpieces during time intervals 2 e. This carbon diffuses from the surface into the workpiece. In order to obtain a more uniform carbon concentration over the thickness of the carburized layer, so-called diffusion phases 2 f are provided between carburization phases 2 e, in which diffusion phases the gas is removed from the furnace space by applying vacuum. Carbon absorbed by the workpiece surface until that time can then diffuse into the workpiece without additional carbon. Phases 2 e and 2 f can be repeated according to the desired carbon distribution. In this exemplary embodiment there are a total of four carburization phases 2 e. This exemplary embodiment contains process steps that would be suitable for thin-walled workpieces, where a relatively small carburization depth is aimed for.

After completion of the last diffusion phase 2 f the inner furnace space evacuated until that point is flooded with hydrogen gas up to an absolute pressure of 10 bars. At the same time the heating is switched off, which has up until then kept the temperature in curve section 1 c constant. Due to hydrogen cooling along curve 1 d the temperature drops rapidly to the ambient temperature. The hydrogen cooling phase is denoted by 2 g. Circulation of the hydrogen gas with an efficient fan inside the furnace assists the heat dissipation. For uniform cooling, which minimizes the displacement of the workpieces during cooling, the hydrogen flow inside the furnace space is deflected several times so that the workpieces are loaded from several sides with the cooling gas. When cooling to almost room temperature is completed, the hydrogen gas in section 2 h is removed from the furnace space until vacuum is achieved. The interior is flooded with nitrogen from 0 bar to ambient pressure to unload the inner furnace space, which is illustrated by curve section 2 i. If the furnace is then opened, air enters the inner space, and the pressure is set to atmospheric pressure. This section of the pressure curve is denoted by 2 k.

In the case of more solid or larger workpieces provision may also be made for quenching to take place initially to a point above the martensite start line, and the temperature is maintained there until the edge temperature and the core temperature of the workpiece have equalized. Further quenching may then be carried out until room temperature is approximately reached.

The method described offers the possibility, in a single chamber vacuum furnace, to achieve cooling rates which otherwise would only be attainable during oil quenching or water quenching. The cooling rate depends on the slope steepness of curve 1 in section 1 d. Although a method is known for cooling workpieces in the hardening shop with hydrogen, this method is not used in practice in single chamber vacuum furnaces because it is considered that the safety problems cannot be solved economically.

Here a new solution for the safety problems is found with the method described. The risk of explosion when filling the interior of the furnace with hydrogen arises because on the one hand ignition sources are present in the furnace, namely the workpieces kept at over 1,000° C., and on the other the hydrogen is present as an oxidizable gas. To eliminate a risk of explosion all oxygen must therefore be kept away from the furnace interior. This is achieved in the method according to the invention in that the air present after the furnace is loaded is almost completely removed, initially in section 2 a, by evacuation. A nitrogen atmosphere is then built up (2 b), which is later also pumped away in the course of the process. Any residual oxygen remaining from the atmosphere or due to degassing the workpieces is then flushed out with the nitrogen. Finally the carburization gas, which does not react with hydrogen, is fed into the furnace. This gas is pumped away repeatedly, which is equivalent to further flushing of the furnace interior.

If in section 2 g the hydrogen is then fed into the interior, there will no longer be any oxygen there. This therefore totally eliminates the risk of explosion. No oxygen enters the furnace interior during the cooling phase either. The hydrogen is then discharged off into the flue by means of discharge valves, and when atmospheric pressure is reached the remaining hydrogen is pumped out of the furnace interior by means of vacuum pumps (section 2 h). Any residual hydrogen present is then diluted by flooding with nitrogen in section 2 i until no further ignitable mixture can be formed in any case. At that time there is also an absence of ignition source in the furnace interior because the entire content of the furnace has been cooled to approximately room temperature. The drive motors of the fans and the heating are without power. The opening of the furnace in section 2 k, for unloading the inventory contained in it, is then entirely non-critical. The air entering during opening finds neither an ignition source nor an adequate concentration of hydrogen to create explosive conditions.

The hydrogen pumped off in section 2 h is discharged through gas tight pipes and vacuum pipes via a flue into the atmosphere outside the operating building. After the hydrogen is pumped off, flue 17 (FIG. 2) is fully flushed with nitrogen to ensure that no hydrogen that could form an ignitable mixture remains in it.

The operating building is shown in more detail in FIG. 2.

FIG. 2 shows, in a diagrammatic representation, an operating building 10 for carrying out the method described above. The operating building is designed as a shop in which a hardening furnace 11 is installed by an intrinsically known method. A storage tank 12 for hydrogen is provided outside the building. Furthermore, a storage tank 13 for gaseous nitrogen is arranged next to a further storage tank 14 for liquid nitrogen. Both storage tanks 12 and 13 for the gaseous supply are connected by connecting pipes 15, 16 to hardening furnace 11.

Hardening furnace 11 is also provided with a flue 17, which leads from the building into the atmosphere. Flue 17 is in this case designed higher than the ridge line of building 10.

On its left front side, hardening furnace 11 has a sealing or closure cover 18 which can be opened for loading and unloading hardening furnace 11. A shaded region 20, in which special precautions are taken against mechanical damage to the outer added components and the pipes, is illustrated behind the plane of sealing cover 18. This region 20 is mechanically protected so that it is not possible to pass through this region in the vicinity of hardening furnace 11 with machines such as fork lift trucks and the like. It is also not possible to pass through region 20 with a gantry crane due to the installation of suitable mechanical devices or electrical precautionary measures which influence the control of the crane. Barriers, guide planks, or even a cage may be provided for this purpose. These safety precautions prevent pipes 15 carrying hydrogen, the associated valve means and pumps, and the hardening furnace itself, from being damaged so that hydrogen can escape inside operating building 10.

Sealing cover 18 of hardening furnace 11 is also provided with a peripheral seal which is sealed safely and hermetically by excess pressure from a protective gas during operation. This prevents leakages from occurring during the transition from vacuum to excess pressure that takes place during operation, as illustrated in curve 2 of FIG. 1.

Because of the safety precautions described it is not necessary to design region 20 so that it is protected from explosions. This results in a reduction in plant and operating costs relative to the concepts of the prior art for hydrogen cooling in the hardening operation.

Finally, FIG. 3 shows hardening furnace 11 in an enlarged representation. The furnace is designed as a single-chamber vacuum furnace with a fan whose axis of rotation is identical to the central axis of the furnace. Furnace door or cover 18 is specially equipped to protect against leaks occurring during the transition between vacuum and excess pressure. This is described in more detail in laid-open specification WO 2004/096427A1, which originates from the same applicant. It should also be mentioned that vertical single-chamber furnaces are being built and that the cooling fan and heat exchanger can also be installed in an external housing which is then connected to the furnace housing.

Initial experiments with the plant and method described have shown that the following lambda values can be achieved in the material core of structural steel bolts: Lambda=0.10 for bolts 20 mm ø×40 mm long Lambda=0.26 for bolts 40 mm ø×80 mm long Lambda=0.72 for bolts 80 mm ø×160 mm long Lambda=1.30 for bolts 120 mm ø×240 mm long

These are values which correspond to quenching in oil.

For a further improvement in the attainable cooling rates provision may be made for a cold water store of several cubic meters of cooling water to be kept at a low temperature of approx. 3° C. - 5° C. for feeding into the heat exchanger for the first 30-60 seconds of the cooling process. This favorably influences the extremely critical time phase of initial quenching from holding temperature 1 c.

The quality of the workpieces is also determined by the distortion that takes place during the hardening process. In order to reduce the distortion problems that still remain in furnaces with gas quenching, the gas flow, with frequent reversals of direction, has been introduced successfully in vacuum furnaces several years ago. Now, this invention provides a new, supplementary solution to this problem.

In the method and with the device according to the invention it is possible to achieve certain specific lambda values. Here the full hydrogen gas pressure is introduced into the furnace for quenching. Depending on the furnace, this pressure may be 10 bars but may also be 20 bars or 40 bars. The cooling rate which must be set to achieve a certain lambda value is controlled by the gas flow rate and ultimately by the rate of circulation inside the furnace. The circulating fan is speed controlled, a control range of 10% of the maximum speed being provided until the full maximum speed is reached. The technical effect of this means that there are three factors of influence for the cooling rate, namely the type of gas, the gas pressure and the gas flow rate. Hitherto, the technical world was of the opinion that these three components were of equal importance. This may be true in terms of the hardness that can be achieved. Because of the distortion of the workpieces, however, differences have been found. For example, the type of cooling gas used affects all surfaces of the workpieces exposed to the cooling gas. The same applies to the gas pressure, which is the same throughout the treatment area of the furnace. However, the flow rate of the cooling gas has different effects on the workpiece surfaces according to how the gas flow is obtained.

The use of hydrogen gas at very high pressure establishes both of the first mentioned points. Now, when lower lambda values than would be possible at full fan power are aimed for, the power of the circulating fan is reduced and the parameter “flow rate of the cooling gas” is adapted to requirements. Due to the gas circulation, which is then slower, a lower distortion of the processed workpiece is obtained than if the fan were to be operated at full power and one of the other parameters were to be changed.

As an alterative to regulating the fan power, a twist throttle or similar means may also be used to influence the flow rate.

The specification incorporates by reference the disclosure of German priority document 10 2005 045 783.5 filed Sep. 23, 2005.

The present invention is, of course, in no way restricted to the specific disclosure of the specification and drawings, but also encompasses any modifications within the scope of the appended claims. 

1. A method for operating a single-chamber vacuum hardening furnace, including the steps of: filling an interior of the furnace with workpieces; sealing the interior of the furnace; evacuating the interior of the furnace; heating the workpieces and maintaining a theoretical temperature; filling the interior of the furnace with hydrogen under high pressure, switching off the heating and circulating the hydrogen for cooling the workpieces; discharging the hydrogen and evacuating the interior of the furnace; filling the interior of the furnace with an inert gas up to approximately atmospheric pressure; and opening the furnace and unloading the workpieces.
 2. A method according to claim 1, which, between the evacuating and heating steps, includes the further steps of: filling the interior of the furnace with an inert gas; convective heating the workpieces with circulation of the inert gas; and evacuating the interior of the furnace.
 3. A method according to claim 2, wherein the inert gas of at least one of said filling steps is nitrogen.
 4. A method according to claim 1, wherein said discharging step comprises conveying the hydrogen out of an operating building via a flue.
 5. A method according to claim 4, wherein said flue is higher than a roof height of the operating building.
 6. A method according to claim 1, wherein during and/or after said step of heating the workpieces and maintaining a theoretical temperature, a gas containing carbon is conveyed at least one time into the interior of the furnace.
 7. A method according to claim 6, wherein said gas containing carbon is pumped out of the interior of the furnace prior to said step of filling the interior of the furnace with hydrogen under high pressure.
 8. A method according to claim 6, wherein said gas is a acetylene.
 9. A method according to claim 8, wherein a carrier gas is added to said acetylene.
 10. A method according to claim 1, wherein a fan is provided for circulating the hydrogen, and wherein the speed of the fan is adapted to be regulated.
 11. A device for carrying out the method of claim 1, and comprising a safety zone that surrounds the hardening furnace, wherein the safety zone is adapted to prevent walking or travelling in the immediate vicinity of the furnace, with the exception of a front side of the furnace that is adapted to be opened by mechanical or electrical means. 